Flexibility, any hubot instance can do a variety of things. If you host within your own infrastructure the sky is the limit.
Initial setup can be tricky with all the tokens to generate. If you are behind a firewall you cannot use the web hook functionality without poking a hole or setting up some sort of repeater.
Read the plugins, install within your own infrastructure, use promises.
I can ask hubot where an application lives within our infrastructure, I can ask hubot to kick off deploys, check on status, and a variety of other things. This allows me to not leave slack, or provide artifacts for others to debug issues.